A delayed coker is a type of coker whose process consists of heating a residual oil feed to its thermal cracking temperature in a furnace with multiple parallel passes. This cracks the heavy, long chain hydrocarbon molecules of the residual oil into coker gas oil and petroleum coke.
Delayed coking is one of the unit processes used in many oil refineries. The adjacent photograph depicts a delayed coking unit with 4 drums. However, larger units have tandem pairs of drums, some with as many as 8 drums, each of which may have diameters of up to 10 meters and overall heights of up to 43 meters.
The yield of coke from the delayed coking process ranges from about 18 to 30 percent by weight of the feedstock residual oil, depending on the composition of the feedstock and the operating variables. Many refineries worldwide produce as much as 2,000 to 3,000 tons per day of petroleum coke and some produce even more.
The flow diagram and description in this section are based on a delayed coking unit with a single pair of coke drums and one feedstock furnace. However, as mentioned above, larger units may have as many as 4 pairs of drums (8 drums in total) as well as a furnace for each pair of coke drums.
Residual oil from the vacuum distillation unit (sometimes including high-boiling oils from other sources within the refinery) is pumped into the bottom of the distillation column called the main fractionator. From there, it is pumped, along with some injected steam, into the fuel-fired furnace and heated to its thermal cracking temperature of about 480 °C. Thermal cracking begins in the pipe between the furnace and the first coke drums, and finishes in the coke drum that is on-stream. The injected steam helps to minimize the deposition of coke within the furnace tubes.
Pumping the incoming residual oil into the bottom of the main fractionator, rather than directly into the furnace, preheats the residual oil by having it contact the hot vapors in the bottom of the fractionator. At the same time, some of the hot vapors condense into a high-boiling liquid which recycles back into the furnace along with the hot residual oil.
As cracking takes place in the drum, gas oil and lighter components are generated in vapor phase and separate from the liquid and solids. The drum effluent is vapor except for any liquid or solids entrainment, and is directed to main fractionator where it is separated into the desired boiling point fractions.
The solid coke is deposited and remains in the coke drum in a porous structure that allows flow through the pores. Depending upon the overall coke drum cycle being used, a coke drum may fill in 16 to 24 hours.
After the first drum is full of the solidified coke, the hot mixture from the furnace is switched to the second drum. While the second drum is filling, the filled first drum is steamed out to reduce the hydrocarbon content of the petroleum coke, and then quenched with water to cool it. The top and bottom heads of the full coke drum are removed, and the solid petroleum coke is then cut from the coke drum with a high pressure water nozzle, where it falls into a pit, pad, or sluiceway for reclamation to storage.
The table below illustrates the wide range of compositions for raw petroleum coke (referred to as green coke) produced in a delayed coker and the corresponding compositions after the green coke has been calcined at 2375 °F (1302 °C):
at 2375 °F
|Fixed carbon, wt %||80 - 95||98.0 - 99.5|
|Hydrogen, wt %||3.0 - 4.5||0.1|
|Nitrogen, wt %||0.1 - 0.5|
|Sulfur, wt %||0.2 - 6.0|
|Volatile matter, wt %||5 - 15||0.2 - 0.8|
|Moisture, wt %||0.5 - 10||0.1|
|Ash, wt %||0.1 - 1.0||0.02 - 0.7|
|Density, g/cm3||1.2 - 1.6||1.9 - 2.1|
|Metals, ppm weight:|
|Aluminum||15 - 100||15 - 100|
|Boron||0.1 - 15||0.1 - 15|
|Calcium||25 - 500||25 - 500|
|Chromium||5 - 50||5 - 50|
|Cobalt||10 - 60||10 - 60|
|Iron||50 - 5000||50 - 5000|
|Manganese||2 - 100||2 - 100|
|Magnesium||10 - 250||10 - 250|
|Molybdenum||10 - 20||10 - 20|
|Nickel||10 - 500||10 - 500|
|Potassium||20 - 50||20 - 50|
|Silicon||50 - 600||50 - 600|
|Sodium||40 - 70||40 - 70|
|Titanium||2 - 60||2 - 60|
|Vanadium||5 - 500||5 - 500|
Petroleum coke was first made in the 1860s in the early oil refineries in Pennsylvania which boiled oil in small, iron distillation stills to recover kerosene, a much needed lamp oil. The stills were heated by wood or coal fires built underneath them, which over-heated and coked the oil near the bottom. After the distillation was completed, the still was allowed to cool and workmen could then dig out the coke and tar.
The uses for green coke are:
The uses for calcined coke are:
There are other petroleum refining processes for producing petroleum coke, namely the Fluid Coking and Flexicoking processes both of which were developed and are licensed by ExxonMobil Research and Engineering. The first commercial unit went into operation in 1955. Forty-three years later, as of 1998, there were 18 of these units operating worldwide of which 6 were in the United States.
There are other similar coking processes, but they do not produce petroleum coke. For example, the Lurgi-VZK Flash Coker which produces coke by the pyrolysis of biomass.